The present invention relates generally to a method and an apparatus for monitoring the shape and size of particles and granules during granulation and coating processes and relates, more particularly, to such a method and apparatus wherein momentary images of the particles and granules are photographed and analyzed.
For the production of granules, various types of granulation equipment and apparatus are commonly employed, depending upon the purpose and desired quality of end products. Two conventional types of granulators are so-called fluidized bed granulators and tumbling-type granulating apparatus. Granules produced by fluidized bed granulators, for example, are generally of irregular shape and of relatively large size, while granules produced by tumbling-type granulators are relatively spherical in shape.
It is known that the size and shape of such granules depend upon the operational conditions under which granulation takes place and may have relatively large fluctuations making it normally difficult to get consistent granule quality. Thus, depending upon the intended purpose and application of the granules, one specific type of granulator may be selected over another for the production of granules with a specific size and shape.
One of the difficulties encountered in any granulating operation is the selection of the operational parameters for the process and the determination of the point in the process at which operation of the granulator will be stopped. Normally, granule properties are monitored during granulator operation by manual material sampling, but the final properties of the granules are checked only after the granulation process is completed.
Various types of sensing devices to monitor the size and shape of granules during a granulation operation have been developed, but most of them only provide for indirect measurement of granule properties. Specifically, one known conventional monitoring device measures the relative dynamic torque of the main mixing element or rotational element in the granulation chamber as a means of indirectly extrapolating granule size and shape. Another device provides a sensing bar or beam with a strain gauge for insertion into a granulation chamber to measure the degree of deviation of the strain gauge signal due to collision with the granules against the sensor bar as an indication of granule size and shape. A third type of device monitors deviations in power consumption of the main mixing element or rotational element in the granulator as a function of the granule size. Another device measures moisture content of granules as indirectly indicative of the size of granules under process. Therefore, direct measurement or monitoring of individual granule size and shape during a granulating process is increasingly demanded.
Methods for direct measurement of granule size and shape by photographic means are disclosed, for example, in Japanese Patent Application Nos. 54-92389, 59-81535, 63-266339, 60-15541, 57-59143, 58-73730 and 4-265142, the first three of which disclose methods for photographing granules falling from a belt conveyor, with lighting being radiated from a direction opposite or perpendicular to the photographing axis. One problem with these methods is that many granules overlap one another leading to incomplete separation of individual granules in the image processing system and resulting in inaccurate measurement. To overcome this problem, granules should be placed on the belt conveyor so as not to overlap each other. A more fundamental problem is that this method cannot be applied to the photographic analysis of granules in the granulator during processing.
In Japanese Patent Application No. 57-59143, the shape of granules traveling on a belt conveyor is monitored by a so-called linear-sensor camera and granule size is measured by frequency analysis, but this method also cannot be applied to the granules in the granulator during processing. In Japanese Patent Application No. 58-73730, stroboscopic photography is utilized to measure granule size in the mixing and granulation process, but this method suffers the same problem of granule overlapping. Japanese Patent Application No. 4-265142 discloses a method for automated photography during the granulation process in a fluidized bed granulator, by taking a granule sample onto a transparent adhesive tape through which a granule image is obtained. The common problem among these various granule photography methods is that they cannot achieve a substantially separated individual granule image during the granulation and coating process, mainly due to granule overlapping problems.